1. Field of the Invention
The present invention relates to a photographing lens and, more particularly, to a high-performance compact photographing lens which is suitable for a compact image sensing apparatus, such as a digital still camera, monitor camera, or PC camera (an image sensing apparatus attached to a personal computer), using an image sensing device such as a CCD (Charge Coupled Device).
2. Description of the Related Art
In recent years, image sensing apparatuses such as digital still cameras for ordinary consumers are rapidly spreading. A digital still camera electrically receives, as image data, a still image formed by a photographing lens through an image sensing device such as a CCD (to be also referred to as a CCD or the like hereinafter) and records the image data in an internal memory or on a memory card. At first, a digital still camera has, as its fascination, immediate image playback and convenience which are unavailable in a silver halide film camera because the liquid crystal monitor can be used as a viewfinder in photographing and also as a playback monitor for a sensed image. However, many digital still cameras employ CCDs with small numbers of pixels. It has been pointed out that a digital still camera generally has a lower image resolution as compared to a silver halide film camera.
However, recent technological development makes it possible to supply inexpensive CCDs or the like with great numbers of pixels. When an image obtained by a digital still camera having a CCD with a large number of pixels is printed in, e.g., a popular size, the image has almost the same resolution as that of an image taken by a silver halide film camera.
A conventional photographing lens for a digital still camera will be examined. The photographing lens receives an image by using a CCD or the like having a large number of pixels. From this viewpoint, the photographing lens has an arrangement similar to that of a photographing lens for a video camera. However, the photographing lens for a digital still camera has a more complex arrangement because a high resolution and image quality are demanded. Additionally, when the image sizes of the CCDs are compared under same conditions, the photographing lens for a digital still camera is often larger than the photographing lens for a video camera.
Characteristics required for a conventional photographing lens for a digital still camera will be outlined below.
(1) Lens Must Cope with High Image Quality
The number of pixels in a CCD or the like is recently 3,000,000 to 4,000,000 even in digital still cameras for general consumers. This number of pixels is about 10 times larger than that of an image sensing element with 350,000 pixels, which is generally used in a video camera, although they have different image sizes and therefore cannot be compared directly. This means that they have such a difference even in aberration correction accuracy (correction difficulty) required for photographing lenses.
Currently, the number of pixels of a CCD or the like is generally increased by decreasing the pixel pitch while suppressing an increase in CCD size as much as possible. For example, a CCD or the like with 1,300,000 effective pixels, which is recently announced as an image sensing device for a digital still camera, has a pixel pitch of about 4.2 xcexcm. Assumed to be twice the pixel pitch, the diameter of the circle of least confusion is 8.4 xcexcm. The diameter of the circle of least confusion of a 35-mm silver halide film camera is probably about 33 xcexcm. As is also apparent from this comparison, the resolution required of a photographing lens for a digital still camera is about four times that for a silver halide film camera. Furthermore, as the number of pixels of a CCD or the like increases, such requirement becomes stricter.
(2) Image-side Telecentricity must be Excellent
Image-side telecentricity is such a phenomenon that the principal ray of a light beam with respect to each image point becomes almost parallel to the optical axis, i.e., almost perpendicular to the image plane after principal ray exits the final surface of the optical system. In other words, the exit pupil position of the optical system is sufficiently far apart from the image plane. The reason why the image-side telecentricity is necessary is as follows. A color filter on the object side of the image sensing surface of a CCD or the like is located slightly far apart from the image sensing surface. If a light beam becomes incident obliquely, it is partially vignetted, and the actual aperture efficiency decreases (shading). The image-side telecentricity is necessary for solving or relaxing this disadvantage. Especially, a recent highly sensitive CCD or the like often has a microlens array immediately in front of the image sensing surface. In this case as well, if the exit pupil is not sufficiently far apart from the image plane, the aperture efficiency becomes low around the image plane. It is considerably significant to obtain excellent image-side telecentricity.
(3) Some Back-focus must be Ensured
In a CCD or the like, a protective glass plate must be arranged in front of the image sensing surface because of the structural requirement. In addition, a space that generally accommodates several optical elements must be ensured between the optical system and the CCD or the like of the photographing lens. Examples of the optical elements are an optical low-pass filter which is inserted to prevent moire and the like, which take place due to the periodical structure of the CCD or the like, and an infrared absorbing filter which is also inserted between the optical system and the CCD or the like in order to decrease the sensitivity of the CCD or the like in the infrared range and make the sensitivity closer to the spectral luminous efficiency. Hence, a photographing lens for a digital still camera requires some back-focus to accommodate such a glass plate and optical elements.
A photographing lens for a digital still camera has the above-described three characteristics (conditions) to be satisfied, unlike a lens for a silver halide film camera. In fact, there are recently indications that the requirement for the xe2x80x9cimage-side telecentricityxe2x80x9d will be relieved by changing the layout of a color filter or microlens array in a CCD or the like, and the requirement for xe2x80x9cback-focusxe2x80x9d will be relieved by using other materials for an optical low-pass filter and the like and radically changing the structure of the CCD or the like.
As the conditions are moderated by these improvements, a development environment for a photographing lens which has excellent characteristics for a digital still camera and advantages in size and cost is being prepared.
The present invention has been made in consideration of the above situation and has, as its object, to provide a compact photographing lens having a high resolution and a small number of constituent elements.
In order to achieve the above object, according to the first aspect of the present invention, there is provided a compact photographing lens wherein an aperture stop is arranged at a position closest to an object, a first lens group, a second lens group, and a third lens group are arranged sequentially from the object side, the first lens group is formed by arranging, sequentially from the object side, a first lens having a positive refracting power and a second lens having a negative refracting power and bonded to the first lens, the second lens being formed from a meniscus lens with a concave surface facing the object side, the second lens group is formed from only a third lens as a meniscus lens which has at least one aspherical refracting surface and a concave surface facing the object side, the third lens group is formed from only a fourth lens as a meniscus lens which has at least one aspherical refracting surface and a convex surface facing the object side, and the photographing lens satisfies conditions (1) to (5):
0.80 less than f/f1,2 less than 1.00xe2x80x83xe2x80x83(1)
xe2x88x920.10 less than f/f4 less than 0.45xe2x80x83xe2x80x83(2)
0.2 less than |f/r2-2| less than 1.6xe2x80x83xe2x80x83(3)
2.9 less than |f/r3-1| less than 4.4xe2x80x83xe2x80x83(4)
2.0 less than f/d1 less than 2.9xe2x80x83xe2x80x83(5)
where
f: focal length of entire system
f1,2: combined focal length of first and second lenses
f4: focal length of fourth lens
r2-2: radius of curvature of image-side surface of second lens
r3-1: radius of curvature of object-side surface of third lens
d1: central thickness of first lens.
According to the second aspect of the present invention, in the photographing lens according to the first aspect, both the third and fourth lenses are made of a resin material and satisfy condition (6):
|f/f3,4| less than 0.35xe2x80x83xe2x80x83(6)
where
f3,4: combined focal length of third and fourth lenses.
Inequality (6) in the second aspect is related to the combined refracting power of the second and third lens groups, i.e., the third and fourth lenses. When both the second and third lens groups are made of a resin material, i.e., plastic, an aspherical surface can be applied. In addition, the cost reduction effect can be expected. As a characteristic feature of a plastic lens, it is readily affected by a change in temperature. However, when the value |f/f3,4| representing the combined refracting power of the second and third lens groups falls within the range of inequality (6), focal point movement in case of a change in temperature can be made sufficiently small.
According to the third aspect of the present invention, there is provided a photographing lens which satisfies, in addition to condition (1) to (5), condition (7):
27 less than xcexd1xe2x88x92xcexd2xe2x80x83xe2x80x83(7)
where
xcexd1: Abbe number of first lens
xcexd2: Abbe number of second lens.
Inequality (7) represents a condition for chromatic aberration correction in the first lens group. When the value xcexd1xe2x88x92xcexd2 is larger than the lower limit value, especially, on-axis chromatic aberration can easily be corrected.
When the photographing lens according to any one of the above aspects is used, a compact image sensing apparatus capable of obtaining a high-quality image can be provided.
As is apparent from the above aspects and their functions, according to the present invention, a compact photographing lens having a high resolution and a small number of constituent elements can be provided.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principle of the present invention are shown by way of illustrative examples.